The drive line shaft connects the transmission output to the differential at the rear wheel on rear drive vehicles. The rotary motion of the transmission output shaft is carried to the differential and from there to the wheel. The drive line shaft must change in length and angle as the wheels move up and down. To allow for these two variations, two or more universal joints and a slip joint are required. The universal joint accommodates a change in the drive angle. The slip joint accommodates a change in the length of the drive line.
In the conventional drive line shaft, the slip joint has outside splines on one shaft and matching internal splines on a mating hollow shaft. The splines cause the two shafts to rotate together and to permit the two to move endwise in relation to each other. This allows changes in the length of the propeller shaft as the rear axles move toward or away from the vehicle frame.
Some vehicles, such as busses, have the engine mounted at the rear. Due to the limited space available, short shafts and universal joints carry the engine power to the rear wheels. It would be desirable to provide the shortest transmission shaft possible without sacrificing the strength of the shaft to support bearing stresses.